In mining and the working of rock walls, forces are released that can cause rock to move into adjacent free space. To prevent this, anchors are built into blast holes. One such adjustable anchor is disclosed in European patent No. 94,908, wherein one end of a tie rod is fastened to the bottom of the blast hole by an expanding element and the other end is provided with a thread, on which a nut is screwed. The nut is adjacent to a washer plate that bears against the wall of the mine.
Adhesive anchors are also known, wherein the tie rod is, throughout its entire length, cemented to the wall of the blast hole by a two-component adhesive. With these anchors, a nut screwed onto the free end of the tie rod only prevents the wall of the hollow space from crumbling by means of a washer plate, thus having to transfer forces that are considerably smaller. The adhesive is first introduced into the blast hole, packed into plastic bags. At its anterior end the tie rod is provided with cutting edges which, when the rod is introduced into the hole, cuts open the bags. By turning the tie rod in the blast hole, both adhesive components are mixed and the mixture is then distributed over the entire dimension and length of the rod. After the adhesive mixture has hardened, the nut can be tightened.
In European patent No. 94,908 the tie rod is made from a glass fiber reinforced, synthetic material. In addition to high sturdiness and low weight, this also has the advantage that the tie rod does not corrode. Difficulties are caused, however, by the transfer of forces from the tie rod to the nut. To overcome this problem a casing is cast onto the end of the tie rod and features a saw tooth shaped thread whose flanks, not facing the tie rod's free end, are considerably steeper than those facing the free end. The casing is secured on the tie rod by saw tooth shaped grooves, which are moulded into the rod and whose axial length decreases toward the free end of the rod in order to prevent any axial displacement. This construction of the free rod end has proven effective, but is quite costly from the point of view of production engineering.
In French patent No. 1,197,548 prestressed steel rods for reinforced concrete are connected to a tension element by casings, featuring a longitudinal slot with an "inverted" saw tooth thread. In its boring, the casing is provided with teeth to improve the frictional connection with the tie rod. The thread, however, is steep and is not suitable for tightening under the influence of a load. To prevent an axial displacement of the casings on the steel rods, the rods are thicker at their ends.